1. Field of the Invention
The present invention relates to a zero IF receiver including a demodulator for deriving amplitude and frequency information.
2. Description of the Related Art
The zero IF technique is a well known method of implementing a narrowband radio frequency receiver, in which substantial parts of the receiver can be constructed in one integrated circuit. The filtering function at RF is implemented at baseband by the use of low pass filters in two quadrature channels "I" and "Q". In order to recover the signal from these two channels, some form of demodulation is required. Typically amplitude would be recovered by means of a "square and add" circuit, which is equivalent to the use of a square law detector in a conventional superhet receiver. Frequency would be recovered by means of a standard FM demodulator. The dynamic range over which the demodulator is required to work is often kept within reasonable bounds by means of an automatic gain control (AGC) loop. However, in some applications, for instance when a short duration signal, for example a pulsed sine wave, is received there is insufficient time for an AGC loop to operate over the full dynamic range of the receiver, say between 70 dB and 80 dB at RF, or 140 dB to 160 dB after amplitude demodulation. This makes the design of an integrated demodulator circuit very difficult, if not impossible, using conventional techniques.